Implement dt.verify_integrity() function

[st-pasha]

This function should check whether the data in the datatable is consistent, and correct it if possible. In particular, the following checks are needed:

• all the checks already performed by the window() function
• check that boolean columns contain only values 0, 1, and NA
• check that string/categorical columns contain valid UTF-8 strings
• check that string/categorical columns have offsets that are monotonically increasing (in abs value) and do not go out-of-range
• check that string data column in a string section is padded with 0xFFs
• check validity of all rollup values
• (to be continued)